The present invention relates to telephone systems and particularly to a device for providing privacy in telephone systems where multiple phones have access to a common telephone line and/or for alerting a user of a phone device connected to the common telephone line when an additional telephone device on the same telephone line is also off-hook.
In many commercial and residential telephone installations there are a number of telephone line operated devices which share a common telephone line. In such installations, various privacy concerns may arise. For example, if someone is talking on one of the telephone devices, another party can pick up a second telephone and eavesdrop on the conversation without the first person being aware. Additionally, if a telephone, modem, fax machine or other telephone device is transmitting information on the telephone line, it is possible for a further telephone, modem, fax machine or other telephone device to connect to the telephone line, thus interfering with the conversation or data transfer taking place on the telephone line. Various devices have been proposed for solving these problems; however the existing devices have various technical or cost disadvantages.
Current anti-eavesdropping and privacy protection devices can generally be broken down into two broad categories--privacy (or exclusion) devices and alerting devices. A privacy device operates by preventing the telephone device that it is connected to from accessing the common line when another telephone device is already off-hook. Privacy devices are generally connected to each telephone device sharing a common line. Alerting devices operate by providing some form of alarm when another telephone device on the common line goes off-hook in addition to the protected telephone device. Alerting devices are usually only connected to the telephone device they are intended to protect.
An example of a privacy device is disclosed in U.S. Pat. No. 5,231,658 issued Jul. 27, 1993, to V. Eftechiou, which discloses a privacy device that can prevent the phone device that it is connected to from accessing a common telephone line if another phone device on the line is already off-hook.
U.S. Pat. No. 4,443,665 issued Apr. 17, 1984 to Alderman also discloses a privacy device which can prevent the telephone device it is connected to from accessing a common line when another telephone device is already off-hook. The Alderman privacy device also includes a hold circuit for placing callers on hold.
A problem with current privacy circuits is that because they must be connected to the telephone device that they are intended to exclude, they can easily be overridden by a potential eavesdropper without notice to the eavesdropee by disconnecting them from the telephone device that the eavesdropper intends to use.
An example of an alerting device can be seen in U.S. Pat. No. 4,230,914 issued Oct. 28, 1980 to P. Korwin et al., which discloses an alerting device for installation in the hand set of a telephone which provides a visual alarm when a subsequent telephone device on the same telephone line goes off-hook. However, the device disclosed in this patent requires an external power source in the form of a battery, and must be connected to the internal circuitry of the telephone, which makes it impractical.
U.S. Pat. Nos. 4,726,048 and 4,941,166 issued Feb. 16, 1988 and Jul. 10, 1990, respectively, to H. H. Waldman et al. disclose an alerting device which cuts off the audio signal to a first off-hook telephone when a further telephone sharing the same telephone line is subsequently taken off-hook. The alerting device in Waldman et al. operates by cutting off the voice path to the first telephone when the voltage across the telephone line drops below a pre-determined threshold. Such a device is inconvenient as it cuts off the voice path to the first user entirely when a second telephone is off-hook, thereby isolating the first user.
U.S. Pat. No. 4,136,266 issued Jan. 23, 1979 to Cohen et al. discloses an alerting device which detects the fact that an extension telephone has been placed in the off-hook condition by detecting "click" signals which are placed on the telephone line whenever any extension telephone is placed either in the off-hook condition or, if it is already in the off-hook condition, when it is placed in the on-hook condition. The "click" signals are defined in Cohen et al. as having distinctive acoustic and electrical properties which can be detected as a function of the frequency of the signal and the voltage level thereof. The Cohen et al. patent provides complex detection circuitry for detecting these click signals and generates an alarm whenever these signals are detected. The external alarm requires that the device be located where it can be seen/heard by the user.
In order to detect these signals, the privacy circuit of Cohen et al. relies on a tuned detection circuit which must be tuned to the specific frequency of the clicks. Since this frequency will vary from telephone to telephone, it requires a manual adjustment of a variable resistor forming part of the tuned circuit. This manual adjustment must be made by the user of the alerting device for each individual installation.
In addition to the above shortcomings, the existing alerting devices do not have the dual ability to act as privacy devices in that they will not prevent a second telephone device from connecting to the common line when a first telephone device is already off-hook. This is particularly a concern when the second telephone device is a modem or fax which will not respond to the same stimulus that a human eavesdropper would. Also, the existing alerting devices are inconvenient in that they do not include a hold function for allowing an incoming call to be placed on hold.
Thus, it is desirable to provide a single device which can act as both a privacy device and an alerting device. Specifically, it is desirable to provide a privacy and alerting device which, when connected to a first telephone device will prevent that telephone device from going off-hook if another telephone device on a common-telephone line is already off-hook, and which will sound an alarm if the first telephone device alone is off-hook and another telephone device subsequently goes off-hook.
It is also desirable to provide a privacy and alerting device which will allow more than one telephone device to access the common line when desired, and to provide a privacy and alerting device which includes a hold function.
It is also desirable to have an eavesdropping alerting device for a telephone system which does not require an external source of power in addition to the telephone line, which makes use of a circuit which is easily constructed, and which makes use of an audio alarm which can be heard in the ear piece of the off-hook telephone devices without cutting off the voice path.